In a system of transmitting data over a carrier, the frequency may be as high as 60 GHz frequency band (millimeter wave) and the baseband signal may also be transmitted over a high operating frequency. In radio communication, the frequency band available for the communication is defined by standards or the like; therefore, the frequency spectrum for the transmission should be within the defined range. Behind the scene circumstances of the definition include that a band is narrowed by suppressing the undesired side band for the baseband signal in order to effectively utilize the radio wave resources.
The available frequency band width is subdivided according to the use and purpose. For example, IEEE 802.15.3c defines a limit on the power spectral density for a transmitter to prevent electrical interference with other channels. Therefore, it is required to obtain a desired transmit signal by filtering and pulse shaping the transmit signal to transform it to meet the defined frequency spectrum.
FIG. 1 is a schematic diagram showing a signal being pulse shaped. FIG. 1 (a) shows pulse shaping on the time axis; and
FIG. 1 (b) shows the frequency spectrum being transformed to desired components by the pulse shaping. It is obtained by sampling the signal at a certain frequency (i.e., at regular intervals). By taking an example of transferring the pulse shape by a single carrier at the rate of 1.728 Gbps, the Digital Analog Converter (DAC) is driven at more detailed threefold frequency and the output waveform is sent. Consequently, the pulse shape forms a very high operating frequency that exceeds 5G samples per second, which is difficult to implement.
Generally, the filter circuit for pulse shaping a signal is a raised cosine Finite Impulse Response (FIR) filter, which can realize the filtering that meets the Nyquist condition and does not produce any interference between signals. This is only available for the ideal case where sufficiently accurate operations can be provided. As a high operating frequency is required for real implementation of the circuitry, the effective number of bits for the DAC is limited; therefore, a highly accurate value cannot be used.
FIGS. 2(a) through 2(e) are diagrams showing a frequency spectrum where the DAC accuracy is from 4 bits through 8 bits. As illustrated, the abscissa represents frequency and the ordinate represents power spectral density. In the figure, the straight line shows the power spectrum defined by the standards, which means that a frequency component with the spectral density exceeding the line should not exist. It is apparent from FIGS. 2(a) through 2(d) that the raised cosine filter using a low accurate DAC from 4 bits to 7 bits does not meet the defined frequency spectrum. As shown in FIG. 2(e), the filter narrowly meets the standard by using the 8-bit DAC, but it is not preferable because the 8-bit DAC is extremely expensive. Here, a pulse shaping method that operates by meeting the standard even with the less than 8-bit DAC by using an implementation other than that of the conventional raised cosine FIR filter, and an implementation for the method are desired. For a digital filter, its accuracy depends on the number of bits in the DAC as mentioned above, but on the other hand, a high-speed operation is required for realizing the wide band.
An analog filter can be considered as a substitute, but its feature might change over the years. In addition, there is variation in the feature of the analog filter at the manufacturing plant, and the feature largely changes as the operating environment (temperature, power supply voltage) changes; accordingly, it is difficult to realize and maintain the accurate Nyquist, which leads a technical problem of lowering the reproducibility.